Metering valves for spray cans are readily available in the marketplace which are constructed to allow a discrete, or pre-proportioned amount of product to be dispensed at each actuation of the valve control push-button. Such metering valves generally comprise a valve body which is associated in a sealed relationship with the mounting cup of the container or can by means of a folded seam or crimped collar. Within the valve body is defined a valve chamber in which an axially slidable valve stem is pushed down by a user against the bias of a valve spring.
Inside the valve chamber there is generally a lower inlet which may include a ring seal adjacent the inlet, where the ring seal is adapted for circumferential engagement with the valve stem, in contact relationship therewith, when the valve stem is in its dispensing position. Upon actuation of the valve stem the metering chamber is isolated from the main reservoir of the container and placed in communication with the outside environment to define a dispensing state. When the stem is released, the valve chamber again communicates with the main reservoir to permit the valve chamber to be recharged and cuts off communication with the outside environment.
A drawback associated with prior art metering valves is that such valves have a tendency to “throttle”. That is, due to a poor or an improper valve design, it is possible for an operator to partially depress the valve stem and establish a product flow path from the interior cavity of the aerosol container through the metering valve and out through a spray button or actuator affixed to the stem of the aerosol valve, prior to the valve stem sealing the inlet to the valve housing, so that product may be continuously discharged out through the aerosol valve. As a result of such “throttling”, the operator is able to dispense a continuous discharge of product from the aerosol container via the metering valve rather than meter a desired amount. This results in the inadvertent discharge of excess product form the aerosol container which is wasteful and generally to be avoided.
Some prior art designs utilize a frictional sealing fit between two plastic valve components to separate the contents of the container from the metering chamber. This arrangement requires that a valve spring, accommodated by the valve, be sufficiently forceful to overcome the interference fit of this sealing device upon the valve closing sequence. Apart from being subject to size and hardness changes due to immersion in the product, this design mandates extremely close tolerances of the mating components and critical alignment of molded parts during the valve assembly operation. Also, insufficient return force, or pre-load, on the valve spring can cause failure of the valve stem to return to an initial closed position after initial depression. Failure to observe these manufacturing tolerances, pre-load and alignment criteria leads to an inaccurate metered spray or a valve which will not “shut-off” and thus result in the total release or dispensing of the entire product contents.
It is to be appreciated that such metering valves must be manufactured so as to accommodate different volumes of product which a manufacturer desires to dispense with each actuation. With the known metering valves, it happens that, where the useful volume of the metering chamber is to be changed, i.e., when it is desired to change the amount of product dispensed at each actuation, the cylindrical valve body must be entirely redesigned and manufactured with such a volume in mind and/or replaced as a whole.
It follows therefore, that in order to change the dispensed proportion of product the valve body must be entirely re-manufactured with the thickness of the valve body walls being adjusted thicker or thinner, and/or the length of the entire metering valve body being changed and the majority of the metering valve components also have to be replaced with others having different dimensions which creates a problem both as regards to production and inventory.
Also, it is well known in the industry that inadequate sealing engagement and material incompatibility between the valve stem and the seal ring, and/or the valve chamber inlet, when the valve is actuated can lead to very undesirable results of inadvertent over-dispensing of the product as the product leaks from the container reservoir through the poor seal between around the valve stem and into the valve chamber during an actuated state of the valve.